1. Field
The present invention relates generally to controlling electro-magnetic radiation, and particularly to apparatus for restricting power and intensity of light in optical sensors.
2. Related Art
Modern optical systems suffer from the lack of visibility near the line-of-sight to bright objects in the sky, such as the sun or the moon. When the sun enters the field-of-view of a camera, the sunlight may saturate and potentially damage optical sensors in the camera. This problem is sometimes solved by means of a coronagraph, which is an apparatus used in astronomy to mechanically block light from a bright star in order to detect a nearby faint source. However, such a complex actively tracking mechanism typically is not applicable to existing commercial cameras and optical systems. A better approach may involve a device that acts as a non-linear optical broadband filter and provides necessary masking effect for sensors at high optical powers and intensities.
The ability of a material or device to lower the transmission of incoming light is known as optical limiting. An ideal limiter exhibits a linear transmission below a threshold and afterwards clamps its output to a constant level. Approaches to achieve this effect have been based on the use of both active and passive devices. Active devices rely on feedback loops, which control an aperture allowing either greater or smaller amounts of light to pass through it. Because of electronics that is required to drive such systems, they are generally too slow, complex and expensive. Passive devices that exploit the nonlinear optical susceptibility of a given substance to incoming light have been considered more suitable, because their response can be ultrafast. This effect can be achieved by one or more of the nonlinear optical mechanisms, such as excited state absorption (ESA), free-carrier absorption (FCA), two photon absorption (TPA), thermal defocusing/scattering, photo-refraction, nonlinear refraction, and induced scattering. Optical limiting can be enhanced by coupling two or more of the nonlinear optical mechanisms. A wide range of materials with various nonlinear optical mechanisms contributing to optical limiting and nonlinear absorption have been investigated, primarily for the purpose of protecting against high power laser beams. To date, no single material has been found that can provide the required protection levels for any of the optical systems against high power lasers.
The requirements for optical limiting against natural high brightness objects, such as the sun and the moon, are even more demanding. Optical intensities, which are required to produce nonlinear effects (ESA, TPA, etc.), can be achieved in high power pulsed lasers, but are far above those that are produced by the sun (or the moon). Passive optical limiters that have been in development for protection against pulsed lasers therefore cannot be used as means for high brightness protection. Thus, a dramatically new approach is needed as an alternative method to optical limiting, being able provide the same functionality.